Background: I do a lot of work with apps at scale, so dealing with really big sets of data, and doing maths on it. With that in mind, I'm biased towards the mat

Background: I do a lot of work with apps at scale, so dealing with really big sets of data, and doing maths on it. With that in mind, I'm biased towards the maths that I use, but I'll try and be broader and as unbiased as I can be. 1. Statistics. There's so much you can do with a really solid understanding of the basics of statistics, it's silly. It's massively helpful with things at scale where you get into difficult caching challenges, systems with multiple data sources and estimating data accuracy, informing based on partial or sampled data... Really useful stuff. 2. Geometry. You'd be amazed how many problems there are that exist in n dimensional space. A good grounding in geometry will let you understand how lots of systems work. For instance, autocompletes tend to be solved nowadays by calculating distances between input shapes and words in n dimensions, where different stopping points add an extra dimension. Also really helpful when you start dealing with complex data systems. 3. Interesting algebra. Mostly if you understand code, you'll pick this up pretty quick. Lots of philosophical similarities. Beyond that, it depends on what fields you want to go in to as you can get really specialised. But whatever you write, knowledge of those three will always be handy.